This research project is designed to study the protein metabolic state of healthy and diseased newborn infants. We utilize the urinary excretion of the muscle specific unusual amino acid 3-methylhistidine to directly measure muscle breakdown. As the amount of creatinine in the urine is directly proportional with the muscle mass of an infant, the ratio of urinary 3-methylhistidine over creatinine describes the amount of muscle breakdown versus total muscle mass. In the first two years of the grant, we have carried out about 500 24 hour 3-methylhistidine excretion and 3-methylhistidine/creatinine ratio measurements in about 120 infants with various diseases. At the same time we have also determined about 300 24 hour Nitrogen-balances, which is presently used as a measure of the protein metabolic state. We have found a very significant correlation in (p equals less than 0.001) between the two metabolic parameters. Presently, we are continuing the research efforts toward the study of the metabolism of mature and premature infants with various clinical conditions, as well as healthy infants, in order to develop 3-methylhistidine/creatinine ratios which are representative of the various metabolic states. We believe that in the near future 3-methylhistidine/creatinine ratio will be directly applied as an exact, relatively quick measure of protein metabolic states in infants. As a clinical tool 3-methylhistidine/creatinine ratio will allow us to establish the need and the necessary rate of administration of parenteral alimentation.